Unbonded post-tensioning system

ABSTRACT

An unbonded post-tensioning system and method are disclosed, as well as concrete structures utilizing an unbonded post-tensioning system. A void is created and maintained through the concrete structure in which at least one stainless steel strand is placed and tensioned. The void containing the cable is maintained unfilled. Strands may thus be removed and inspected, and reinserted or replaced as necessary. The invention has the advantages of reducing corrosion and permitting non-destructive inspection and replacement of individual strands.

FIELD OF THE INVENTION

[0001] The present invention relates to a post-tensioning system for concrete, and more particularly to an unbonded post-tensioning system wherein the post-tensioning tendons are removable and replaceable for inspection and maintenance.

BACKGROUND OF THE INVENTION

[0002] Post-tensioning of concrete structures has been known in the art for many years. In post-tensioning, tendons are placed through a concrete structure and then tensioned using hydraulic jacks in order to place the concrete structure under compression. Examples of various post-tensioning systems are shown in, for example, U.S. Pat. Nos. 3,225,499, 5,271,199, and 5,299,445.

[0003] One area of concern in post-tensioned structures is corrosion of the tendons, which results in degradation in the strength of the structure. In general, a filler material such as grout or grease has been used in the prior art in order to attempt to protect the tendons against corrosion. Because this is a pervasive problem in the art, many different solutions have been proposed, for example in U.S. Pat. No. 4,773,198 a system is proposed that includes corrosion resistant sheaths and screw closures for the sheaths.

[0004] In spite of various attempts to address corrosion problems with tendons, a feasible solution has not been arrived at. The magnitude of the problem posed by corrosion in post-tensioning systems is illustrated by recently discovered difficulties with the Sunshine Skyway in St. Petersburg, Fla. In this structure, the post-tensioning tendons were thought to be protected by plastic conduits filled with grout. However, over the only fourteen year life of the bridge, a number of tendons have failed and many others are damaged due to corrosion. Because the tendons are sealed in grout, they cannot be removed or replaced. In this instance new holes have been drilled in the road bed to insert separate reinforcing rods. Additionally, one of the hollow bridge support pillars was filled with concrete. As will be appreciated, addressing such problems in this manner and after corrosion has begun can lead to great expense and potential degradation of the structure.

[0005] There therefore exists a need in the art for a post-tensioning system that can not only resist corrosion, but that can also permit inspection of individual tendons to determine their status without destruction of the surrounding structure or the tendons themselves.

SUMMARY OF THE INVENTION

[0006] Thus, according to a preferred embodiment of the invention, a post-tensioning system for concrete structures comprises a tubular structure configured and dimensioned to provide a continuous void through a concrete structure, first and second anchor members disposed at opposite ends of the tubular structure, and at least one stainless steel strand extending through the tubular structure and secured under tension between the anchor members. The anchor members are mounted to close and maintain the void with the void being unfilled such that the strand may be easily removed and replaced for inspection and maintenance.

[0007] In a further aspect of the invention, a method for post-tensioning concrete structures is provided. According to the method, steps include assembling a continuous tubular structure within a concrete structure form, the continuous tubular structure opening at opposite ends of the form, pouring concrete into the form and permitting it to cure while maintaining a void through the continuous tubular structure, inserting at least one stainless steel strand through the void, tensioning and anchoring said at least one stainless steel strand at opposite ends of the concrete structure, and maintaining the void unfilled. The strand may be inserted before or after the concrete is poured. In this manner, the method may further include periodically removing at least one stainless steel cable, inspecting the cable, and re-tensioning and anchoring said cable. The cable may be replaced if necessary.

[0008] In yet another aspect of the invention a post-tensioned concrete structure comprises a poured concrete member, a tubular structure defining a void through the concrete member and at least one tensioned stainless steel strand passing through the void and anchored at opposite ends of the structure. Once again, the void is preferably unfilled such that the cable may be removed and inspected. Preferably, anchor members are secured to the stainless steel strand at each end of concrete structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other advantages of this invention will be better appreciated by reference to the following detailed description considered in connection with the drawings, in which:

[0010]FIG. 1 is a cross-sectional view of one end of a post-tensioning system according to the present invention;

[0011]FIG. 2 is a cross-sectional view of a socket for use in connection with the present invention;

[0012]FIG. 3 is a partial side view of a duct for use in connection with the present invention;

[0013]FIG. 4 is an end view of an assembled anchor according to the present invention;

[0014]FIG. 5 is a cross-sectional view through line V-V in FIG. 4; and

[0015]FIG. 6 is a cross-sectional view of a structure according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] As illustrated in FIG. 1, post-tensioning system 10 according to a preferred embodiment of the present invention includes duct 12 received in socket 14. The duct and socket create a passageway through subsequently poured concrete through which post-tensioning strands 16 are placed. Strands 16 are anchored using anchor 18 employing wedges 19 as described in greater detail below. The duct, socket, strands and anchor together make up a post-tensioning tendon.

[0017] In order to protect anchor 18 against corrosion, cover 20 may be employed in a preferred embodiment. In one embodiment cover 20 is a flexible boot made of a material such as heavy rubber. Cover 20 is anchored to the concrete by retaining ring 22 and bolts 24. Alternatively, covers made of other materials, including poured concrete or steel caps, may be employed.

[0018] According to a preferred embodiment of the invention, strands 16 comprise a twisted seven-wire stainless steel cable. Each wire may be about 0.2 inches in diameter, such that the strand has an overall diameter of about 0.6 inches. Preferred stainless steels include ASTM A 368, types 316 or 304. The twisted wires may have a helical pitch of 8 to 9 inches. Persons of ordinary skill in the art may select alternative materials and sizes based on the teachings herein without departing from the scope of the invention. Utilizing stainless steel strands according to the present invention reduces or eliminates corrosion such that the use of grout, grease or other filler materials are not required for protection of the cables. Thus, as described below, strands may be removed, inspected, and reassembled/replaced as part of regular maintenance of the system.

[0019] Socket 14 is shown in greater detail in FIG. 2. Socket 14 may comprise central tubing member 28 onto which end plates 30 and 32 are welded. In an exemplary embodiment tubing member 28 may comprise a 3¾ inch OD, 0.180 inch wall thickness AISI 1020 tubing. Tubing member 28 may be about 9 to 10 inches long. End plates 30 and 32 may comprise 1 inch thick ASTM A36 steel plates approximately 6 to 7 inches square. Plate 32 has a central hole 34 sized to receive duct 12 as shown in FIG. 1. Plate 30 is provided with a larger central opening 36, sized to provide clearance for strand 16 passing therethrough as shown in FIG. 1.

[0020] Duct 12 is shown in detail in FIG. 3. In a preferred embodiment, duct 12 is a 2½ inch OD, 19 gauge, AISI 1010 galvanized steel tube. Alternatively, other materials such as plastic pipe may be used. At one end an enlarged bell 26 is provided so that adjoining sections may be slip-fit together. Duct 12 may be provided in any convenient length. In a further alternative embodiment duct 12 is not used. In such an embodiment, the void is maintained through the concrete without the duct, for example by drilling through hardened concrete.

[0021] As shown in greater detail in FIGS. 4 and 5, wedges 19 clamp strands 16 into anchor 18. Wedges 19 are conically shaped, preferably from AISI 8620 case-hardened steel. In one preferred embodiment, each wedge is approximately 1⅛ inch in length, having a large end diameter of approximately 1.25 inches and a small end diameter of approximately 0.8 inches. The central bore through each wedge 19 is approximately 0.6 inches in diameter and lined with sharp spiral threads at approximately 23 threads per inch. Alternatively, appropriately spaced knurls may be used, for example at a pitch of {fraction (1/19)} and height of about 0.046 inches. As apparent in FIG. 4, two half wedges 19 make up a wedge assembly. The half wedges have a thickness which preferably leaves a parting line opening 38 between the half wedges 19 when they are compressed around the tendon. In a preferred embodiment the width of opening 38 is approximately {fraction (3/32)}-⅛ of an inch. In the illustrated embodiment, seven strands (each made up of seven twisted wires) are used to form a seven-stranded, parallel-lay post-tensioning cable or tendon.

[0022] Anchor 18 includes a number of openings corresponding to the number of strands to be used in the system. As best shown in FIG. 5, openings 40 include a tapered portion mated with wedges 19 and a straight portion permitting each strand 16 to pass therethrough. In a preferred embodiment, the wall of the tapered portion of openings 40 has a pitch of about 1:5 to 1:8. A person of ordinary skill in the art may design suitable combination of wedges and openings to compress and secure the strands based on the teachings provided herein.

[0023] A system according to the invention may be utilized as follows. Prior to pouring concrete structure, the duct and sockets are placed in the concrete forms, for example as shown in FIG. 1. Concrete is then poured and permitted to dry. Once the concrete has set, strands 16 are passed through the duct and socket. Alternatively, the strands may be placed at an earlier point in the process. The strands are then inserted through anchor 18 and tensioned utilizing hydraulic jacks as known in the art. It will be appreciated that FIG. 1 illustrates one end of the post-tensioning system according to the invention and that the same parts will typically be provided on an opposite end of the structure to be post-tensioned. Once the strands are tensioned with a hydraulic jack to an appropriate force level, wedges 19 are placed around the strands and the jacks are released. The elastic force of the strands draws the wedges into the tapered holes of anchor 18 securing the system together and providing the pre-stressing force to the concrete. In order to permit removal as described below, a suitable extra length of strand is left extending from the anchor at at least one end.

[0024] It will be appreciated that in the present invention, because no grout or other filler is necessary in order to protect the cables, individual cables may be removed, inspected and reinserted or replaced. Such inspection may be accomplished as follows: Strands 16 are re-tensioned using a hydraulic jack. In this manner, the wedges may be removed and the strand withdrawn. Once a strand is removed, it may be inspected for corrosion or other degradation and replaced or reinserted as appropriate. The strands may be reinserted or replaced by following the procedure for initial assembly. As will be appreciated, the present invention has the advantages of reducing corrosion and permitting non-destructive inspection and replacement of individual strands or cables.

[0025] The present invention also permits modification of structures with relative ease as compared to prior post-tensioning systems. For example as illustrated in FIG. 6, an original structure, such as a bridge deck, may have comprised a central concrete member, such as member 100. Member 100 may comprise a cantilevered, hollow box construction. In a typical large bridge this may be about 83 feet wide, with an upper flange thickness of about 45 inches and the walls of the hollow box portion 102 being about 30 inches thick. Such a structure is provided with post-tensioning system 10 as previously described.

[0026] In this instance, the structure has been increased in width by adding T-sections 104 to the ends of member 100. In one example, the T-sections may have a width of about 20 feet. By utilizing post-tensioning system 10 according to the present invention, this increase is relatively easily accomplished. The original post-tensioning strands spanning member 100 have been de-tensioned and removed. The new T-sections 104 are prefabricated or cast-in-situ with ducts (not shown) positioned to mate with the ducts in original member 100. T-sections 104 are placed in position adjacent member 100 and new strands of appropriate length are passed through the extended ducts and re-tensioned. Other suitable connections may be made as required for structural soundness in the particular application. The present invention thus permits integral post-tensioning of modified structures without a complete rebuilding of the structure.

[0027] Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications and/or substitutions are possible, in particular with specific dimensions and materials, without departing from the scope and spirit of the invention as recited in the accompanying claims. 

What is claimed is:
 1. A post-tensioning system for concrete structures, comprising: a tubular structure configured and dimensioned to provide a continuous void through a concrete structure, said void being unfilled; first and second anchor members disposed at opposite ends of said tubular structure; and at least one stainless steel strand extending through said tubular structure and secured under tension between said anchor members, said anchor members being mounted to at least substantially close and maintain said void.
 2. The post-tensioning system according to claim 1, further comprising a plurality of stainless steel strands secured to each anchor member and tensioned therebetween.
 3. The post-tensioning system according to claim 2, further comprising a cover member disposed over each anchor member.
 4. The post-tensioning system according to claim 3, wherein said cover member comprises a flexible rubber boot.
 5. The post-tensioning system according to claim 3, wherein said cover member comprises a poured concrete cap.
 6. The post-tensioning system according to claim 3, wherein said cover member is a steel cap.
 7. The post-tensioning system according to claim 1, wherein said tubular structure comprises first and second socket members adjoining said anchor members, said socket members communicating through at least one duct member.
 8. A method for post-tensioning concrete structures, comprising: assembling a continuous tubular structure within a concrete structure form, the continuous tubular structure opening at opposite ends of the form; pouring concrete into the form and permitting it to set while maintaining a void through said continuous tubular structure; inserting at least one stainless steel strand through said void; tensioning and anchoring said at least one stainless steel strand at opposite ends of the concrete structure; and maintaining said void unfilled.
 9. The method of claim 8, further comprising: periodically removing said at least one stainless steel strand; inspecting said strand; and re-tensioning and anchoring said strand.
 10. The method of claim 9, wherein said re-tensioning and anchoring further comprises replacing said strand with a new stainless steel strand.
 11. The method of claim 8, wherein plural strands forming a cable are tensioned together.
 12. The method of claim 8, further comprising: removing said at least one stainless steel strand; positioning a concrete member with a void therethrough defined by a second tubular structure adjacent to and aligned with the tubular structure of the removed stainless steel strand to define an increased length void; inserting at least one stainless steel strand into said increased length void, said strand having a length corresponding to said increased length; and tensioning and anchoring said at least one strand at opposite ends of the concrete structure including said concrete member.
 13. A post-tensioned concrete structure, comprising: a concrete member; a tubular structure defining a void through said concrete member; at least one tensioned stainless steel strand passing through said void and anchored at opposite ends of the structure, wherein said void is unfilled.
 14. The post-tensioned concrete structure according to claim 13, further comprising anchor members secured to the at least one stainless steel strand at each end of concrete structure.
 15. The post-tensioned concrete structure according to claim 14, further comprising a plurality of stainless steel strands passing through said tubular structure.
 16. The post-tensioned concrete structure according to claim 13, further comprising: a second concrete member positioned adjacent said concrete member; and a second tubular structure defining a void through said second concrete member, wherein said tubular structures mate together to define a continuous void therethrough and said at least one tensioned stainless steel strand passes through both concrete members and is anchored at opposite ends each to one of said members.
 17. A post-tensioning system for concrete structures, comprising: at least one stainless steel strand configured and dimensioned to extend through a void defined in the concrete structure; and first and second anchor members configured and dimensioned to be disposed at opposite ends of said at least one stainless steel strand and to hold said at least one strand under tension while bearing against said concrete structure, said anchor members further configured to at least substantially close and maintain said void.
 18. The post-tensioning system according to claim 17, further comprising a tubular structure for defining said void.
 19. The post-tensioning system according to claim 18, wherein: said tubular structure comprises first and second socket members and at least one duct member; and said socket member being configured to adjoin said anchor members and communicate through said at least one duct member.
 20. The post-tensioning system according to claim 17, further comprising a plurality of stainless steel strands securable to each anchor member and tensionable therebetween. 